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We report here a series of new triphase catalytic materials based on Gemini surfactant exchange forms of Na-montmorillonite clay (MMT) from smectite clay family. Earlier, we started with intercalation of Gemini surfactants using hectorite clay. Despite successful intercalation reaction, hectorite has a lower cation exchange capacity than MMT and its utilization in industry is not as significant. Gemini surfactants represent a new class of surfactants.  They are made of two amphiphilic moieties connected at the level of the head groups or very close to the head groups by a spacer group, as schematically represented in Fig. 1. 

Fig. 1.              Schematic representration of  Gemini (dimeric) Surfactant.

Gemini surfactants are remarkably different in solution properties such as critical micelle concentration (CMC), surface tension, viscosity, etc., from those of conventional surfactants or quaternary alkylammonium salts.  We have synthesized several Gemini surfactants comprised of two N-alkyldimethylammonium bromide groups joined together by an alkyl spacer and with a general formula Cn-Cx-Cn, where n = number of carbons in the free N-alkyl chain and x is the number of carbon atom in the spacer group (e.g. C8-Cx-C8, C12-Cx-C12, C16-Cx-C16, and C18-Cx-C18 where X = 2, 4, 6, and 8).  The Gemini surfactants have been characterized by NMR prior to intercalation reaction.  Several Gemini sufactant-MMT clays have been utilized as a solid phase in triphase catalytic system converting n-butyl bromide to n-butyl chloride.  In this triphase system, we use Gemini-MMT clay complexes as solid phase while water and toluene are two other liquid phases.  In this system, the substrate resides in organic phase while the nucleophile is in the aqueous phase.  Also, all intercalated complexes were studied by x-ray powder diffraction to measure the basal spacing of the clay complexes before kinetic studies.    During intercalation reaction, Gemini surfactant salts in excess of the montmorillonite clay cation exchange capacity (CEC) has been added, and upon the exchange reaction, the intercalates have been washed a few times with water to remove the excess Gemini salts from the clay and air-dried prior to catalytic reaction.  The chlorination of 1-bromobutane was selected as a suitable nucleophilic displacement reaction to demonstrate the effectiveness of Gemini-montmorillonite clay intercalates as triphase catalysts.  Pseudo-first order kinetics was observed for the chlorination reaction.  This is similar to the behavior of conventional monomeric surfactants-clay intercalates that have been extensively studied by several authors.  Also, our recent results on MMT clay has been similar to our earlier studies with hectorite, however, we have seen somewhat higher catalytic activities with MMT clay intercalates.  This might be related to charge distribution on the clay surface as well as its CEC.    

It is interesting that all the reaction mixtures have formed a uniform emulsion.  However, this emulsion was broken with low-speed centrifugation (<2000 RPM) and sometimes on a long time storage.  Similar emulsion formation has also been observed for conventional surfactant-clay system.  Based on literature, organoclays can form thin, membrane like assemblies of platelets at the liquid-liquid interface of an oil/water type of emulsion.  Therefore, the reagents in the emulsified liquid phases are readily transferred to the interface of the clay assemblies for facile reaction.  We have observed various catalytic reactivities in Gemini surfactant-MMT system depending on the Gemini surfactant structure.  The variation in catalytic activity may be attributed to the type of chain length and the spacer group as well as the basal spacing of the clay and the way the surfactants are assembled within the clay interlayer gallery spaces.  Also, the structure of these ordered assemblies depends in part on the length of the alkyl chains and the charge density of the clay layers.  In the case of Gemini surfactants, because of the presence of two cationic centers there are some choices in which the surfactants can attach to the clay interlayer.  The surfactant can attach to either one or two negative clay layer charges along the same surface, it can act as a bridge between the two layers or only one of the two charges need be directly attach to the clay layer while the other existing as an ion pair.  However, due to non-homogeneous charge density distribution in natural MMT clay, a combination of the aforementioned structural assemblies can also be possible.  This factor not only can affect the expected d basal spacing, but it may also change the hydrophilic/hydrophobic character of the Gemini surfactant-clay intercalates.  Therefore, catalytic activity of the Gemini surfactant-clay intercalates may vary depending on the Gemini surfactant structure.  Considering near infinite possibility that exist to generate Gemini surfactants using conventional amphiphilic moieties and any type of spacer groups of the desired structure opens a vast opportunity for exploring an efficient catalysts in a triphase catalytic system.   

I believe the impact of this interdisciplinary research at SIUE has been enormous.  In short, many undergraduate students have gained valuable experience working on different aspects of this research area from learning how to purify natural clays to various Gemini syntheses, NMR/XRD characterization, running triphase catalytic reactions and kinetics.  Several abstracts and posters have been presented in regional as well as national ACS meetings.  Currently, the PI is writing two papers for publication in referred journals which will have an important impact on the PI's career.